This invention relates to apparatus and a method for thermally removing coatings and/or impurities from materials, particularly from materials which are particularly suited to batch processing. In particular the present invention relates to a development of the type of oven described in the applicants International Patent Application published as WO 01/98092 A1, the content of which is hereby incorporated by reference in its entirety.
There is an increasing requirement to recycle materials such as aluminum, magnesium and other metals and non-metals. Often such materials will be coated in paint, oil, water, lacquers, plastics, or other volatile organic compounds (V.O.C.s) which must be removed prior to re-melting the materials. For materials which are capable of being processed at relatively high temperatures without melting, such impurities are typically removed using a thermal process which is sometimes known as de-coating. Such thermal de-coating processes can also be used to dry and/or sterilize materials prior to remelting.
For example, aluminum is often used in the production of beverage cans which are typically coated in paint, lacquers and/or other V.O.C.s. Before used beverage cans (U.B.C.s) or scrap material produced during the manufacture of beverage cans be melted down for recycling, any coatings or other impurities must be removed in order to minimize metal loss.
Thermal de-coating, however, is not limited in application to aluminum but can be used to clean or purify any metal or non-metallic materials which are capable of withstanding the temperatures present in the thermal de-coating process. Thermal de-coating can be used to de-coat or purify, for example, magnesium or magnesium alloys, or titanium or titanium alloys.
Known thermal de-coating processes involve exposing the material to be treated to hot gases in order to oxidize the coatings and/or impurities which are to be removed. This exposure takes place in a closed environment in which the temperature and oxygen content of the hot gases can be controlled. Temperatures in excess of 300 C are required to remove most organic compounds and an oxygen level in the range of 6% to 12% is normally required.
If the temperature and oxygen levels of the hot gases are not carefully controlled the decoating process can result in an uncontrolled operation which may be very dangerous.
The material will usually be shredded before treatment and it is important for effective de-coating that all the surfaces of the shredded material are exposed to the hot gases. If this does not occur then the treatment becomes less effective and, in the case of U.B.C.s in particular, a black stain may be left on the surface of the treated material. It is also desirable for the material to be agitated during the treatment to physically remove lose coatings or impurities from the material.
At present there are three main systems which are used for thermal de-coating, these are:
1. Static Oven
In a static oven, the material is stacked on a wire mesh and hot gases are recirculated through the oven to heat the material to the required process temperature.
This arrangement is not efficient because the hot gases do not come in to contact with the materials that are enclosed within the stack of materials on the mesh. As discussed previously, it is important in de-coating that all the surfaces of the materials being treated are exposed to the hot gases. Also there is no agitation of the material being treated.
2. Conveying Oven
This system uses a mesh belt conveyor to transport materials for treatment through an oven. Hot gasses are passed through the material on the belt as it passes through the oven. The problems with this method are as follows:
The depth of materials on the belt limits the process. The materials are stacked, causing similar problems to those found with the static oven in which materials at the centre of the stack do not come into contact with the hot gases.
There is no agitation of the materials, so loose coatings are not removed.
The conveyor belt life is short.
The materials have to be constantly fed.
The process is not suitable for low volume or continuously changing product.
3. Rotating Kiln
A large kiln is inclined to the horizontal so that material fed or charged into the kiln at its highest end travels towards the lowest end, where it is discharged, under the influence of gravity. The kiln is rotated so that material within the kiln is agitated and a flow of hot gases is provided to heat up the material as it travels through the kiln. A number of problems are associated with this method:
The material has to be constantly fed.
The process is not suitable for low volume or continuously changing product.
The continuous process requires air locks at both ends, materials charge end and materials discharge end.
The kiln requires a rotating seal leading to a high level of maintenance.
WO 01/98092 A1 describes a pivotable or tiltable oven that overcomes many of the disadvantages of the previously known apparatus and methods for thermal de-coating. For a detailed description of the construction and operation of the oven, the reader should refer to WO 01/98092 A1. However, briefly, the oven has a charging portion for receiving material to be treated and a changeover portion. Incorporated within the changeover portion is a heat treatment chamber through which a stream or flow of hot gasses can be passed. The oven is pivotally moveable between a first position in which the changeover portion is higher than the charging portion and a second position in which the charging portion is higher than the changeover portion. The arrangement is such that the oven can be repeatedly moved between the first and second positions so that material within the oven falls from one portion to the other portion, passing through the stream of hot gasses in the heat treatment chamber. A method of using the apparatus is also disclosed.
The above known oven has the advantage that it can be used to treat comparatively low volumes of material in a batch process. A further advantage is that by controlling the movement of the oven, the material being treated can be brought into and out of the heat treatment chamber at will, enabling the oven to be operated safely without having an excessive amount of VOC released that could cause self sustained process heating (also known as an autothermic process). This controlled movement ensures that the V.O.C.s are released in a controlled manner and allows a fine degree of control of the treatment process.
In the preferred embodiment of the oven described in WO 01/98092 A1, the main after burner is located within an afterburner chamber integral with the body of the oven and, as the oven is pivoted between the alternative positions, the after burner chamber moves with the oven.
The oven described in WO 01/98092 A1 has been found to work well and thereby providing a commercially and technically acceptable means of thermally de-coating relatively low volumes of materials. However, it has been found that the location of the main afterburner chamber integral with the body of the moving oven is not ideal for certain applications.
It is an object of the present invention to provide an improved oven in which the problems of the known oven are overcome or at least reduced.
Thus, in accordance with a first aspect of the invention there is provided apparatus for thermally de-coating and/or drying coated and/or contaminated materials, the apparatus comprising:
at least one support;
an oven mounted to the or each support and adapted for receiving material to be treated;
each oven being moveable between a first position in which a first portion is generally higher than a second portion and a second position in which the second portion is generally higher than the first portion, such that, in use, the oven can be repeatedly moved between the first and second positions so material within the oven falls from one portion to the other portion;
characterized in that the or each oven does not include an integral afterburner chamber and the apparatus further comprises at least one afterburner for generating a stream of hot gasses and conduit means for directing the stream of hot gasses into a treatment zone of the oven; and exhaust means for returning the gasses to the at least one afterburner.
The treatment zone may be located in the first or second portion of the oven, or partially in each portion, dependent upon the material to be treated and its topology.
The apparatus according to the invention may comprise a single oven and a single afterburner; a single oven and a plurality of afterburners; a plurality of ovens and a single afterburner or a plurality of ovens and a plurality of afterburners.
It is an advantage of the apparatus according to the invention that the provision of an afterburner which is does not pivot with the oven provides a simpler and therefore less expensive solution to the problem of thermally removing coatings and/or impurities from materials.
In accordance with a second aspect of the invention, there is provided a method of thermally de-coating and/or drying coated and/or contaminated materials comprising:
providing an apparatus comprising at least one support and an oven mounted to the or each support and adapted for receiving material to be treated; each oven being moveable between a first position in which a first portion is generally higher than a second portion and a second position in which the second portion is generally higher than the first portion;
placing the material in the or each oven; repeatedly moving the or each oven between said first and second positions so material repeatedly falls from one portion to the other portion;
characterized in that the or each oven does not include an integral afterburner chamber and the apparatus further comprises at least one afterburner for generating a stream of hot gasses and conduit means for directing the stream of hot gasses into a treatment zone of the or each oven and exhaust means for returning the gasses to the at least one afterburner.